Growth and differentiation factor-8 (GDF-8), also known as myostatin, is a secreted protein and is a member of the transforming growth factor-beta (TGF-β) superfamily of structurally related growth factors, all of which possess physiologically important growth-regulatory and morphogenetic properties (Kingsley et al. (1994) Genes Dev., 8: 133-146; Hoodless et al. (1998) Curr. Topics Microbiol. Immunol., 228: 235-272). Similarly to TGF-β, human GDF-8 is synthesized as a 375 amino acid long precursor protein. The precursor GDF-8 protein forms a homodimer. During processing the amino-terminal propeptide is cleaved off at Arg-266. The cleaved propeptide, known as the “latency-associated peptide” (LAP), may remain noncovalently bound to the homodimer, thereby inactivating the complex (Miyazono et al. (1988) J. Biol. Chem., 263: 6407-6415; Wakefield et al. (1988) J. Biol. Chem., 263: 7646-7654; Brown et al. (1990) Growth Factors, 3: 35-43; and Thies et al. (2001) Growth Factors, 18: 251-259). The complex of mature GDF-8 with propeptide is commonly referred to as the “small latent complex” (Gentry et al. (1990) Biochemistry, 29: 6851-6857; Derynck et al. (1995) Nature, 316: 701-705; and Massague (1990) Ann. Rev. Cell Biol., 12: 597-641). Other proteins are also known to bind to mature GDF-8 and inhibit its biological activity. Such inhibitory proteins include follistatin and follistatin-related proteins (Gamer et al. (1999) Dev. Biol., 208: 222-232).
An alignment of deduced amino acid sequences from various species demonstrates that GDF-8 is highly conserved throughout evolution (McPherron et al. (1997) Proc. Nat. Acad. Sci. U.S.A., 94: 12457-12461). In fact, the sequences of human, mouse, rat, porcine, and chicken GDF-8 are 100% identical in the C-terminal region, while in baboon, bovine, and ovine they differ only by 3 amino acids. The zebrafish GDF-8 is the most diverged; however, it is still 88% identical to human.
The high degree of conservation suggests that GDF-8 has an essential function. GDF-8 is highly expressed in the developing and adult skeletal muscle and was found to be involved in the regulation of critical biological processes in the muscle and in osteogenesis. For example, GDF-8 knockout transgenic mice are characterized by a marked hypertrophy and hyperplasia of the skeletal muscle (McPherron et al. (1997) Nature, 387: 83-90) and altered cortical bone structure (Hamrick et al. (2000) Bone, 27 (3): 343-349). Similar increases in skeletal muscle mass are evident in naturally occurring mutations of GDF-8 in cattle (Ashmore et al. (1974) Growth, 38: 501-507; Swatland et al. (1994) J. Anim. Sci., 38: 752-757; McPherron et al. (1997) Proc. Nat. Acad. Sci. U.S.A., 94: 12457-12461; and Kambadur et al. (1997) Genome Res., 7: 910-915). Studies have indicated that muscle wasting associated with HIV-infection is accompanied by an increase in GDF-8 expression (Gonzalez-Cadavid et al. (1998) Proc. Nat. Acad. Sci. U.S.A., 95: 14938-14943). GDF-8 has also been implicated in the production of muscle-specific enzymes (e.g., creatine kinase) and proliferation of myoblast cells (WO 00/43781). In addition to its growth-regulatory and morphogenetic properties, GDF-8 is thought to be also involved in a number of other physiological processes, including glucose homeostasis in the development of type 2 diabetes, impaired glucose tolerance, metabolic syndromes (e.g., syndrome X), insulin resistance induced by trauma, such as burns or nitrogen imbalance, and adipose tissue disorders (e.g., obesity) (Kim et al. (2001) BBRC, 281: 902-906).
A number of human and animal disorders are associated with functionally impaired muscle tissue, e.g., muscular dystrophy (including Duchenne's muscular dystrophy), amyotrophic lateral sclerosis (ALS), muscle atrophy, organ atrophy, frailty, congestive obstructive pulmonary disease, sarcopenia, cachexia, and muscle wasting syndromes caused by other diseases and conditions. To date, very few reliable or effective therapies have been developed to treat these disorders.
There are also a number of conditions associated with a loss of bone, which include osteoporosis and osteoarthritis, especially in the elderly and/or postmenopausal women. In addition, metabolic bone diseases and disorders include low bone mass due to chronic glucocorticoid therapy, premature gonadal failure, androgen suppression, vitamin D deficiency, secondary hyperparathyroidism, nutritional deficiencies, and anorexia nervosa. Currently available therapies for these conditions work by inhibiting bone resorption. A therapy that promotes new bone formation would be a desirable alternative to these therapies.
Thus, a need exists to develop new therapies that contribute to an overall increase of muscle mass and/or strength and/or bone density, especially, in humans.